In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic field pattern is developed into an electrostatographic toner pattern by contacting the field pattern with an electrostatographic developer containing an electrostatographic toner. If desired, the latent electrostatic field pattern can be transferred to another surface before such development. Although such techniques are typically used for black and white reproductions such as copying business correspondence, they are capable of forming a variety of single color or multicolor toner images.
A typical method of making a multicolor copy involves trichromatic color synthesis by subtractive color formation. In such synthesis successive latent electrostatic images are formed on a substrate, each representing a different color, and each image is developed with a toner of a different color and is transferred to a support (receiver). Typically, but not necessarily, the images will correspond to each of the three primary subtractive colors (cyan, magenta and yellow), and black as a fourth color, if desired. For example, light reflected from a color photograph to be copied can be passed through a filter before impinging on a charged photoconductive layer so that the latent electrostatic image on the photoconductive layer corresponds to the presence of yellow in the photograph. That latent image can be developed with a yellow toner and the developed image can be transferred to a support. Light reflected from the photograph can then be passed through another filter to form a latent electrostatic image on the photoconductive layer which correspond to the presence of magenta in the photograph, and that latent image can then be developed with a magenta toner and transferred to the same support. The process can be repeated for cyan (and black, if desired).
In the systems described previously herein, the toner images may be provided on a support such as paper, film, plastic or glass to which they are permanently fixed. A common technique for fixing such toner images to a support involves employing thermoplastic polymeric toner particles which include a colorant to form the unfixed or unfused image and then fusing the particles to the support by the application of heat and pressure thereto. A suitable method involves passing the support with the toner particles thereon through a pair of opposed rolls, one a heated fuser roll and the other a non-heated or heated backup roll.
It is known to use toner fusing processes to provide toner images having certain enhanced characteristics. For example, U.S. Pat. No. 4,913,991, issued Apr. 3, 1990, describes a process for preparing glossy electrostatographic toner images which the patent indicates presents a pleasing appearance to a viewer, particularly where such images are multicolor toner images.
In the process described in U.S. Pat. No. 4,913,991 a toner image is formed on a recording sheet and fused by passing the sheet between a heat application roll coated with a fluorine-containing resin and a pressure application roll. The toner image has rheological characteristics such that its loss tangent (tan .delta.) is in the range of 1.70 to 3.00 at a storage elastic modulus (G') of 10.sup.5 dyne/cm.sup.2. U.S. Pat. No. 4,913,991 indicates that the aforementioned loss tangent ranges are critical to obtaining acceptable fused toner images having the required gloss and presents comparative data to illustrate this point. The process described in U.S. Pat. No. 4,913,919 is adequate to provide glossy toner images but, it does have some drawbacks. For example, the process is not as flexible a process as would be desired since it is limited to use with toner images having the aforementioned limited range in loss tangent values.
It is also known in the prior art that it is a problem to provide colored toner images having maximum color saturation and, in colored transparencies, maximum chroma or color clarity. Color desaturation in a colored toner image can result from light scattering or multiple reflections within the toner image. This is a problem in reflection color copies but it is particularly troublesome in subtractive color images in transparencies where such light reflection can also result in color shifts upon projection and a failure to faithfully reproduce the colors of the original image. For example, bright yellow in an original image may appear as a muddy yellow. The term often used in the prior art to describe the quality of an image projected by a transparency is "chroma" and high chroma refers to a faithful reproduction of the original colored image while low chroma refers to less than faithful or inaccurate representation of the original colored image. U.S. Pat. No. 4,791,447, issued Dec. 13, 1988, addresses the problem of providing glossy opaque toner images and high chroma transparencies using a fusing system comprising three roll members which cooperate to form a pair of roll nips.
In light of the previous discussion, it is obvious that it would be desirable to have a fusing method capable of providing a wide variety of electrostatographic toner images exhibiting enhanced gloss. Likewise, it would be desirable for such fusing method to have the capability of providing color transparencies exhibiting excellent color clarity, i.e. high chroma. This invention provides such a fusing method.